As one of the fastest changing glaciers in Antarctica, Thwaites Glacier has become a symbol of climate change. The enormous speed with which it is retreating is apparently due to different melting processes on its underside, as recent observations indicate. A look under the ice with theIcefin underwater vehicle has shown that under much of the ice shelf, the ice is melting much more slowly than previously thought. In cracks and crevices, on the other hand, melting is much faster.
Two studies published Feb. 15 in the journal Nature bring more clarity to what’s going on beneath the Thwaites Glacier ice shelf. Until now, researchers have assumed that the ice shelf is melting extensively from below because of warmer seawater. However, after a team from the British Antarctic Survey, Cornell University and other institutions used the Icefin robot to look beneath the floating ice shelf, these assumptions must be corrected.
Their results show that although melting under the ice shelf has increased, it is occurring much more slowly than many computer models currently estimate. In particular, this is because a layer of less saline water between the bottom of the ice shelf and the ocean below reduces the rate of melting along flat areas.
However, the team was surprised to find that melting had formed a stair-like topography on the underside of the ice shelf, where melting occurs more rapidly, as well as in crevasses in the ice.
“Our results are a surprise but the glacier is still in trouble. If an ice shelf and a glacier is in balance, the ice coming off the continent will match the amount of ice being lost through melting and iceberg calving. What we have found is that despite small amounts of melting there is still rapid glacier retreat, so it seems that it doesn’t take a lot to push the glacier out of balance,” says Dr. Peter Davis of the British Antarctic Survey, lead author of one of the two studies.
The new data were collected as part of the MELT project, one of eight subprojects of the International Thwaites Glacier Collaboration. The MELT team observed beneath the eastern Thwaites Ice Shelf the area along the baseline where the glacier meets the ocean.
Dr. Davis’ team compared the measurements Icefin took between the borehole and the baseline about two kilometers away with measurements from five other locations under the ice shelf. They found that over a nine-month period, the ocean near the baseline became warmer and saltier. However, melt rates at the ice base averaged “only” two to five meters per year, much lower than estimated by current computer models, which assume annual ice loss of 14 to 32 meters.
The second team, led by Dr. Britney Schmidt, associate professor at Cornell University and lead author of the second study, used Icefin to collect more data under the ice and capture images of the ice and seafloor. They found that the stairs or terraces at the bottom of the ice shelf are melting rapidly, as are the crevasses. In particular, melting in crevasses is important because water can flow through them and heat and salt can be transferred into the ice, widening crevasses and cracks.
It is these newly observed faster melting processes that could be significant for the loss of ice from Thwaites Glacier, especially as larger cracks extend across the ice shelf and become the main trigger for its collapse.
“These new ways of observing the glacier allow us to understand that it’s not just how much melting is happening, but how and where it is happening that matters in these very warm parts of Antarctica. We see crevasses, and probably terraces, across warming glaciers like Thwaites. Warm water is getting into the cracks, helping wear down the glacier at its weakest points,” says Dr. Schmidt.
Dr. Davis rates the new findings as “neither good news nor bad news” in terms of sea level rise, he told Nature. “The glacier is still moving as quickly as it ever has been,” Dr. Davis said.
However, the results contribute significantly to a better understanding of the Thwaites Glacier’s response to climate change. The new data could fill gaps in computer models, removing uncertainties in global sea level rise projections.
Julia Hager, PolarJournal
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